A comparatively recent development in the petroleum arts is the use of C.sub.5 -C.sub.7 methyl alkyl ethers, especially methyl tertiary butyl ether (MTBE) and tertiary amyl methyl ether (TAME) for enhancing gasoline octane. It is known that alkyl tert-alkyl ethers can be prepared by reacting a primary alcohol with an olefin having a double bond on a tertiary carbon atom, thus methanol reacts with isobutylene and isopentenes (2 methyl 1-butene or 2 methyl 2-butene) to form respectively methyl tert-butyl ether (MTBE) and methyl tert-amyl ether (MTAE). The reaction is selective for tertiary olefins so that it constitutes a valid process for their removal from olefinic streams in which they are contained together with linear unreactive olefins. The reaction has an equilibrium which is favorable to the synthesis of the ether as the reaction temperature is lowered, in accordance with the reactions negative enthalpy.
It is known that the reaction is catalyzed by Lewis acids (aluminum trichloride, boron trifluoride), mineral acids (sulfuric acid) and organic acids (alkyl and aryl sulfonic acids, ion exchange resins). Particularly suitable for the task are ion exchange resins in their acid form and it is known that the best results are obtained by means of macroreticular resins of the type "Amberlyst 15". By means of such last named catalysts it is possible to reach thermodynamic equilibrium within industrially acceptable contact times in the temperature range of 50.degree.-60.degree. C.
U.S. Pat No. 4,262,145 to Selwitz et al. discloses the catalytic reaction of a branched olefin such as isobutylene, 2-methylpentene-2, 2-methylbutene-2, and 2,3-dimethyloctene-2 with a lower alkanol such as methanol to form a mixed ether such as methyl tert-butyl ether. The catalyst disclosed is silicotungstic acid.
A process is also known for manufacturing ethers from linear mono-olefins, thereby augmenting the supply of high octane blending stock for gasoline. The lower molecular weight ethers, such as methyl isopropyl ether, are in the gasoline boiling range and are known to have a high blending octane number.
U.S. Pat. No. 4,714,787 to Bell et al., incorporated herein by reference in its entirety, provides a catalytic process for selectively reacting one or more linear monoolefins with a primary or secondary lower molecular weight alcohol to form the corresponding ether. The active acidic catalyst component for the process is selected from the group consisting of sulfonated ion-exchange resins and crystalline silicates having a pore size greater than 5 A.U. Of the crystalline silicates, those preferred include crystalline zeolites having a silica to alumina mol ratio greater than about 12. In a particularly preferred embodiment, methanol and propylene are reacted to selectively form methyl isopropyl ether (MIPE).
A preferred feedstock for the manufacture of MTBE and TAME in petroleum refinery operations is the light hydrocarbon stream from FCC operations. These streams are rich in C.sub.4 + tertiary olefins such as isobutylene. However, they also contain significant amounts of linear olefins plus linear and branched paraffins. The linear olefins, particularly propylene and 1-butene, are not etherified in the prior art MTBE processes. Conventionally, these linear unreacted olefins are carried through the process and separated downstream. In this regard they represent a burden on the volumetric effectiveness of the etherification process, providing no contribution to the production of ether-rich high octane gasoline.
Accordingly, it is an object of the present invention to provide a process for the enhanced conversion of hydrocarbon streams containing mixed branched and linear olefins to ether-rich high octane gasoline.
It is another object of the present invention to provide an integrated process for the sequential conversion of tertiary olefins and linear olefins to lower alkyl ethers.
Yet another object of the instant invention is to provide an integrated process for converting hydrocarbon feedstock comprising mixed olefins and paraffins to high octane ethers and gasoline.